Redmine

I have someone trying to update a ticket in our redmine system every time she tries to update some of her tickets she gets this message:

Internal error
An error occurred on the page you were trying to access.
If you continue to experience problems please contact your Redmine administrator for assistance.
If you are the Redmine administrator, check your log files for details about the error.

I have checked the production.log and I found this below:

Started GET "/issues/427" for 10.123.2.3 at 2012-07-11 17:27:32 -0400
Processing by IssuesController#show as HTML
Parameters: {"id"=>"427"}
DEPRECATION WARNING: :confirm option is deprecated and will be removed from Rails 4.0. Use ':data => { :confirm => 'Text' }' instead. (called from app_views_issues_action_menu_html_erb___288876616_33107184 at C:/redmine-2.0.3/app/views/issues/_action_menu.html.erb:6)
Rendered issues/_action_menu.html.erb (15.6ms)
Rendered issues/show.html.erb within layouts/base (93.7ms)
Completed 500 Internal Server Error in 359ms

ActionView::Template::Error (incompatible character encodings: UTF-8 and ASCII-8BIT):
76:
77: <p><strong><%=l(:field_description)%></strong></p>
78: <div class="wiki">
79: <%= textilizable @issue, :description, :attachments => @issue.attachments >
80: </div>
81: < end >
82: <= link_to_attachments @issue %>
app/views/issues/show.html.erb:79:in `_app_views_issues_show_html_erb__80551109_32868444'
app/controllers/issues_controller.rb:118:in `block (2 levels) in show'
app/controllers/issues_controller.rb:115:in `show'

What is the cause and how do I fix this?